Material color is darker in Kd in .mtl file than it is in blender
Like Troy said it's a color management convertion.
In cycles colors aren't bounded to 0/1 range, this is scene referred datas.
In .mtl files colors are stored in a 0/1 range, the display referred datas.
Look the Wiki for description of Wavefront .obj, scroll down to the section "Material template library" and subsection "Basic materials"
It's said that : "Color definitions are in RGB where each channel's value is between 0 and 1."

So with that in mind you couldn't expect a 1/1 between Cycles internal values and exported bounded values. Plus I don't know if you're aware that Hex values are gamma corrected in Blender.
It looks like there is a transfere curve, I guess an EOTF curve, see here for more details : the-importance-of-terminology-and-srgb-uncertainty, the whole article is worth reading but there is a section about the sRGB EOTF transfer curve.